This invention relates to apparatus and a method for curing photosensitive coatings. More particularly it relates to apparatus and a method that utilize a high intensity light source to create a beam of light that is reflected and focused, filtered and redirected to cause the ultraviolet rays emitted by the light to impinge on a photosensitive coating on a moving substrate in a manner to cure the coating, while avoiding damage to the substrate due to undesirable heat rays from the light.
The mechanisms of ultraviolet curing for photosensitive coatings are well known and understood. The emitter of ultraviolet light most commonly used is a medium pressure mercury vapor lamp, which provides a broad band of power in the 250-400 nm. range. The lamp may be doped by the addition of certain metal halides or other substances in order to provide a relatively higher spectral output at certain frequencies. The majority of such lamps in commercial use today fall within the 100-300 watt per linear inch power rating. It is well known that as the lamp wattage per inch increases in medium pressure lamps the total proportion of ultraviolet rays to total radiant output increases, although there is a tendency for the spectral output to shift to higher wavelengths. As a result of this phenomenon, care must be taken in selecting lamp output, as there will necessarily be a trade off between increasing lamp power, which will effect a greater cure rate, and sacrificing shorter wavelengths, as this will result, in some cases, in reduced top surface cure. The manner or degree of curing of photosensitive coatings by ultraviolet light is a function of several factors, including the specific photo initiator used and its extinction coefficient at a particular wavelength.
The removal of non-ultraviolet radiated and/or heat energy in the associated heating of a substrate in the curing process for photosensitive coatings and problems related thereto have been approached and resolved in many different fashions.
U.S. Pat. No. 3,950,650 to Robert W. Pray et al. and U.S. Pat. No. 4,563,589 to H. D. Scheffer et al. disclose the use of air cooling for ultraviolet curing lamp systems.
U.S. Pat. No. 3,766,377 to K. Junginger et al. discloses an incandescent spotlight system that includes an incandescent lamp, a reflector disk, heat filter, cover plate which seals the light aperture, a Fresnel lens which refracts the light rays to form a parallel beam, and a non-explosive pressurized gaseous coolant system.
Various arrangements have been proposed and utilized to rotate lamp assemblies so as to direct light away from a substrate or to interpose a shutter device between a lamp and a substrate under certain conditions to avoid overheating of the substrate. Such arrangements are shown in U.S. Pat. No. 3,831,289 to R. Knight, U.S. Pat. No. 3,894,343 to R. W. Pray, et al. and U.S. Pat. No. 4,220,865 to S. Silverman.
In similar fashion various arrangements have been proposed and utilized for liquid cooling of apparatus for applying radiant energy. For example, U.S. Pat. No. 2,380,682 to E. W. Boerstler discloses the use of a water cooler around an incandescent lamp and the use of both a liquid filter and a solid filter for the purpose of filtering long wave infrared rays. U.S. Pat. No. 4,000,407 to C. H. Keller and U.S. Pat. No. 4,221,177 to R. M. Mason each discloses a water system associated with a mercury vapor lamp for filtering and cooling purposes.
Another arrangement for delivering relatively cold ultraviolet light to a substrate is shown in U.S. Pat. No. 4,048,490 to H. H. Troue. In this patent dichroic filters are used to absorb undesired infrared light in a high intensity light-source system and direct relatively cold ultraviolet light on a substrate having a coating to be cured.
Still another arrangement that makes use of intense radiant energy to dry printed sheets is disclosed in U.S. Pat. No. 3,159,464 to H. C. Early et al., wherein a high pressure, mercury arc lamp or carbon arcs are used as a source of radiant energy. The radiant energy is of high intensity having a wavelength predominantly within a particular range, and the intensity of the radiation must exceed, for example, one kilowatt per square inch of printed surface.